Merge pull request #6 from dtolnay/checkout

Update example workflow in readme to actions/checkout@v3

README.md

@@ -14,7 +14,7 @@ jobs:
     name: cargo test
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - uses: dtolnay/rust-toolchain@stable
       - run: cargo test --all-features
 ```